WILDFIRESIdentifying “Double-Hazard” Zones for Wildfire in the West

Published 10 February 2022

Rapidly growing communities in the American West’s forests and shrublands are nestled in zones where local soil and plant traits amplify the effect of climate change on wildfire hazards and lead to bigger burns.

Some plants and patches of Earth withstand heat and dry spells better than others. A new Stanford University study shows those different coping mechanisms are closely linked to wildfire burn areas, posing increasing risks in an era of climate change.

The results, published Feb. 7 in Nature Ecology and Evolution, show swaths of forest and shrublands in most Western states likely face greater fire risks than previously predicted because of the way local ecosystems use water. Under the same parched conditions, more acreage tends to burn in these zones because of differences in at least a dozen plant and soil traits.

The study’s authors set out to test an often-repeated hypothesis that climate change is increasing wildfire hazard uniformly in the West. “I asked, is that true everywhere, all the time, for all the different kinds of vegetation? Our research shows it is not,” said lead author Krishna Rao, a PhD student in Earth system science.

‘Double-Hazard’ Zones
The study arrives as the Biden administration prepares to launch a 10-year, multibillion-dollar effort to expand forest thinning and prescribed burns in 11 Western states.

Previous research has shown that climate change is driving up what scientists call the vapor pressure deficit, which is an indicator of how much moisture the air can suck out of soil and plants. Vapor pressure deficit has increased over the past 40 years across most of the American West, largely because warmer air can hold more water. This is a primary mechanism by which global warming is elevating wildfire hazards.

The new analysis, which comes from the lab of Stanford ecohydrologist Alexandra Konings, suggests vapor pressure deficit is rising fastest in areas where plants are especially prone to drying out. The combination of highly sensitive, tinder-dry plants and a faster-than-average increase in atmospheric dryness creates what the authors call “double-hazard” zones.

The 18 newly identified double-hazard zones lie within regions that have seen a disproportionately rapid rise in burn area with every uptick in vapor pressure deficit over the past two decades. Ranging in size from a few hundred to nearly 50,000 square miles, they’re concentrated in eastern Oregon, Nevada’s Great Basin, central Arizona’s Mogollon Rim and California’s southern Sierra Nevada, where recent wildfires have destroyed thousands of giant sequoia trees that had survived fires for hundreds of years.